Building a collection starts innocently enough. A pretty seashell plucked from the sand. A plush giraffe to commemorate a trip to the local zoo. Stones gathered on a walk through the woods. Before you know it, your child's room is overtaken by boxes of pinecones, albums of baseball cards, a herd of plastic animals, or whatever else strikes his fancy. To an adult, all that stuff might look like clutter, but to a child, collecting is an exciting exercise in creating a world all his own -- one that he has control over. "Having a collection helps kids this age fit in and bond with their friends," says Kathleen Camara, Ph.D., associate professor of child development at the Eliot-Pearson Department of Child Development at Tufts University, in Medford, Massachusetts. It gives your child a starting-off point for conversations ("Hey, wanna see my Beanie Boos?") as well as an easy way to socialize (like trading Squinkies). Still tempted to return that box of rocks to its natural habitat? Then read on to find out more about the important lessons your young collector is gathering.

Becoming an Expert

Any kid who's tried to explain the significance of a rare trading card can tell you that there are some things grown-ups just don't get. And that's exactly how kids like it. "It's empowering for a child to know more about a subject than his parents or other adults do," says Dr. Camara. Your child's driven to learn more so he can continue to be an authority on the topic.

Adding Responsibility

The same child who can't recall where she left her jacket and who has to be reminded a billion times to pick up her room will astound you with her devotion to caring for her prized possessions. "Collecting instills a sense of pride and ownership in kids," says Monica Cardoza, author of Child's Play: Enriching Your Child's Interests, From Rocket Science to Rock Climbing, Stamp Collecting to Sculpture. You can foster this enthusiasm by helping your child find a good way to display her treasures -- i.e., on shelves or in albums, tins, craft boxes, or small hammocks for cradling stuffed animals.

Making Social Connections

Whether your child is showing off his Justin Bieber cards at a playdate or telling a school friend about an addition to his dragon menagerie, collectibles bring peers with common interests together. Collections that require trading, like baseball cards, also teach the art of friendly negotiation. If the stakes aren't high, let him make his own deals. "What might not seem like a fair trade to you could be acceptable to him," Dr. Camara says. If he later regrets trading, he'll learn to think through decisions more carefully next time.

Fostering School Skills

If your child collects state quarters or American Girl dolls, it's a pretty good bet that she knows exactly how many are in her collection and which ones she hopes to get next. Don't tell her, but all those hours she puts into counting, sorting, labeling, categorizing, and organizing are actually exercising her math skills. Reading also gets a boost: Dinosaur fans may invest time learning more about the days when those creatures roamed the earth -- and that means trips to the library or museums. Collectors can also go online with your supervision to websites like usmint.gov/kids/campcoin (which has tips on collecting and caring for coins) or stamps.org/young-philatelists (for ways to start a stamp collection).

Learning About Money

Asking children to pay for collectibles out of their own funds is a smart way to teach them about saving and budgeting, as well as helping them learn delayed gratification, says Dr. Camara. "The point shouldn't be to complete a collection or to accumulate mass quantities. Each addition should have meaning, and having to plan for and earn the funds for that next coveted item makes owning it all the more special."